Table 1 Distribution of demographic and clinical characteristics according to zygosity in a nationwide Danish population of twins with PsA (Moll and Wright criteria)
Characteristics of PsA casesMonozygotic(n = 12)Dizygotic(n = 39)Difference(95% CI)
Pairwise characteristics
Mean age in 2002 in years (range)50 (27, 74)51 (27, 79)1.0 (−6.3, 8.4)
Mean discordance time of arthritis (range)*8.7 (1, 16)13 (1, 42)4.4 (−2.2, 11)
Mean time of shared environment (range)*19.0 (14, 30)19 (14, 29)0.7 (−0.8, 2.2)
Characteristics of twin individuals
Female proportion29%52%24% (−6%, 46%)
Mean age at onset of psoriasis (range)29 (11, 50)26 (2, 54)3 (−5.5, 11)
Mean age at onset of arthritis (range)37 (20, 59)36 (18, 63)0 (−8, 8)
Mean number of joints involved†4.53.70.9 (−1.1, 2.9)
Proportion with axial disease‡17%7%9% (−8%, 38%)
Proportion with nail involvement71%50%21% (−8%, 44%)
Proportion HLA-B13 positive0%11%11% (−12%, 25%)
Proportion HLA-B27 positive21%5%16% (−3%, 42%)
  • PsA, psoriatic arthritis.

  • *Calculated on the pairs that were concordant for response. †Number of joints with current or previous arthritis. ‡Axial disease was defined as inflammatory back pain and definite unilateral or bilateral sacroiliitis on x ray (see text).